This paper develops a posteriori estimates for global-in-time, nonoverlapping domain decomposition (DD) methods for heterogeneous diffusion problems. The method uses optimized Schwarz waveform relaxation (OSWR) with Robin transmission conditions on the space-time interface between subdomains, and a lowest-order Raviart–Thomas–Nédélec discretization in the subdomains. Our estimates yield a guaranteed and fully computable upper bound on the error measured in the space-time energy norm of [19, 20], at each iteration of the space-time DD algorithm, where the spatial discretization, the time discretization, and the domain decomposition error components are estimated separately. Thus, an adaptive space-time DD algorithm is proposed, wherein the iterations are stopped when the domain decomposition error does not affect significantly the global error, allowing important savings in terms of the number of domain decomposition iterations while guaranteeing a user-given precision. Numerical results for a two-dimensional heat equation are presented to illustrate the efficiency of our a posteriori estimates and the performance of the adaptive stopping criteria for the space-time DD algorithm.